The present invention relates to spirometers and more particularly to a recording spirometer capable of performing a single expiratory breath test and multiple breath expiratory and inspiratory tests such as a measurement of maximum voluntary ventilation or of tidal volume.
In general, spirometers measure physiological parameters associated with breathing. These parameters include lung volumes and the rates at which air is inhaled and exhaled. Among the tests which spirometers typically perform are the forced vital capacity test, slow vital capacity test, the measurement of the subdivisions of lung volume such as tidal volume and expiratory reserve volume and the measurement of maximum voluntary ventilation. In the forced vital capacity test, the subject takes a deep breath and then exhales as rapidly as possible into the spirometer so that the volume of exhaled air as a function of time is measured. In some cases, this exhalation is followed by a maximum inhalation. In the maximum voluntary ventilation test the subject continuously exhales and inhales at a maximum rate for a specified period of time, the spirometer recording the rates of outflow and inflow as a function of time.
Spirometers are known in which a piston moves within an expansible chamber as exhaled air flows into the apparatus. The piston may carry a card or other recording medium which is marked by a pen travelling at a uniform rate across the card so that the combination of motion of the piston and the pen creates a graph of expiratory flow as a function of time. With these devices, however, the measurement of inspiratory as well as expiratory volumes cannot be performed since such a measurement requires bidirectional motion of the piston so as to accommodate a subject who inhales a larger volume of air than he originally exhaled into the spirometer. Spirometers are also subject to inaccuracies, oftentimes due to the manner of supporting the piston for motion within the spirometer housing. Furthermore, it is often desired to perform two or more different tests and to record the results on the same card using pens of different colors. In the known recording spirometers it has been necessary to remove one pen and insert a pen of a different color before performing a later test. This change of pen required recalibrating the position of the newly inserted pen with respect to the recording medium before the beginning of the next test.
It is therefore an object of the present invention to provide a spirometer which is highly accurate and reliable and yet of simple and inexpensive construction.
It is a further object of this invention to provide a spirometer which is capable of performing not only a single breath expiratory test but also the measurement of both expiratory and inspiratory volumes.
A still further object of this invention is a recording spirometer which permits the use of different colored marking instruments without necessitating recalibration as colors are changed.
It is yet another object of this invention to provide a spirometer in which the components exposed to a subject's exhaled air may be easily removed for cleaning and sterilization.
Other objects, features and advantages of this invention will be pointed out in what follows.